Recently, a translucent alumina sintered body has been widely utilized not only in ornament, jewelry and craftwork articles, but as dental materials such as an orthodontic bracket and a mill blank for artificial denture. As its use is extended to such dental materials, the improvement of mechanical properties such as fracture toughness in addition to aesthetic nature based on translucency become an important issue in the translucent alumina sintered body. In particular, since a need for a translucent ceramic bracket characterized by coloring is increased recently, the enhancement in toughness of a translucent alumina sintered body characterized by coloring (hereinafter, translucent colored alumina sintered body) is desired.
Conventionally, as artificial jewelry such as ruby and sapphire, a translucent colored alumina has been produced by Verneuil method, Czochralski method or the like. However, since monocrystals are obtained in such methods, work for a machining of such monocrystals in practical use is required.
To decrease work due to the machining, a method of mixing an alumina powder with transition metal oxides such as chromium oxide, cobalt oxide, and iron oxide, and molding/sintering the mixed powder had been invented (Patent documents 1-6). For example, Patent document 1 discloses a method of mixing an alumina powder with cobalt oxide, nickel oxide, chromium oxide, manganese oxide, and so on, and sintering the mixture under hydrogen or vacuum atmosphere. Furthermore, Patent document 2 discloses a method for producing a translucent colored alumina sintered body by a hot isostatic pressing (HIP) using transition metals such as iron oxide, titanium oxide, vanadium oxide, nickel oxide, chromium oxide and cobalt oxide. Using these methods, translucent colored alumina sintered bodies having color such as blue, green, yellow and pink are obtained.
However, to date, methods for producing a translucent colored alumina sintered body were based on production methods under hydrogen or vacuum atmosphere (for example, Patent document 7), or methods using HIP (for example, Patent document 8), and translucent colored alumina sintered bodies produced by these methods had fracture toughness as low as 3-4 MPa·m0.5 (Patent document 8). Therefore, high value in fracture toughness suitable for uses which require mechanical properties was not obtained.
Regarding the enhancement in toughness of an alumina sintered body, there are reports such as the introduction of different phases (Patent document 9, and Non-Patent document 1) and the anisotropic grain growth of alumina grains (Patent documents 10 and 11). Using these methods, high value in fracture toughness is obtained, but translucency does not appear. The reason for this is considered that the introduction of different phases causes light scattering at interfaces of different phases, and also a sintered body texture containing anisotropic grains, which can be formed by said conventional method, lowers translucency (Patent document 12).
Thus, to date, a translucent colored alumina sintered body having both high fracture toughness and translucency has not been obtained.